Best gearbox oil

I have never been particularly impressed with the gearchange on my 2003 six speed passat tdi, finding it easy to beat the synchromesh when changing up the in the lower gears, particularly when cold. I had to have the differential bearings replaced at 43,000 miles and mentioned it at that time, but no faults were found.
I have been looking at various forums and oil viscosity figures and wondered if using something other than the VW recommended G52 might improve matters. I thought of using Castrol SMX-S and wonder if anyone has had/solved this problem with either this oil or some other.
Many thanks.